1. Field
Disclosed herein is a process for reclaiming spent selenium filter mass containing an inert material, following take-up of mercury by a substance which contains selenium and is present in the filter mass. Selenium is present in spent selenium filters, both in unspent active filter mass, mainly as elemental selenium, and in reacted filter mass as mercury selenides.
2. Description of Related Art
Selenium filters are being used for taking up gaseous elemental mercury, normally in small amounts, from gases and contain a filter mass consisting of an active substance of elemental selenium, selenium sulphide or some other active selenium compound which is capable of reacting with elemental mercury. The main portion of the filter mass consists of a carrier of an inert material, which may consist of silica, aluminium oxide, or a ceramic material. Such a filter, which has been in general use for various purposes for about thirty years, is disclosed in U.S. Pat. No. 3,786,619 which also teaches a suitable method for making the filter.
In use of the filter, the active substance is gradually consumed, forming mercury selenide, and after operation over a period of time the performance of the filter will be reduced so much that its ability to take up mercury and perform the cleaning is no longer adequate. The filter mass is then replaced with fresh filter mass and the spent filter mass must be disposed of in a suitable manner. Such disposal is costly in itself and also requires payment of waste tipping fees. In addition, the disposal often causes problems for several other reasons, e.g., environmentally, and both the carrier material and the active substance are of considerable value. As indicated initially, the used filter mass contains some unspent selenium-containing active substance, while the remainder of the selenium content of the active substance has reacted with mercury to form mercury selenide, which is a very stable compound.
Typically, the spent filter mass may contain 1 to 5% by weight of Hg and may still contain 3 to 5% by weight of unspent elemental selenium.
This being so, a reprocessing method for reclaiming the active substance and/or the carrier material is highly desirable. It is also desirable to be able to separate mercury contained in the filter mass from the inert carrier so that the amount of material that has to be deposed disposed of is as small as possible, e.g. to reduce the costs for the deposition disposal.
Previously, methods have been proposed for processing mercury-containing waste at relatively high temperatures and in the presence of selenium to remove the mercury of the waste in the form of gaseous selenides. In such a method, primarily developed for used button-type batteries, which is described in EP 0655794, the batteries are treated in a rotary furnace at about 800° C. in the presence of selenium to evaporate mercury in the form of selenide, thereby making the batteries harmless.
Such prior art methods for the destruction of mercury-containing materials are not useful or even feasible for processing spent selenium filter masses, particularly so if the filter masses are to be reclaimed, because capture of mercury existing as elemental mercury, Hg0, and as selenide, HgSe, will be problematic. Separation of the selenium for reclaiming will also be troublesome.